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Snake C. Jones and Kirk W. Pomper

Kentucky State Univ. (KYSU) emphasizes research on developing alternative, high-value crops and sustainable agriculture methods for use by limited-resource farmers. Since 1990, KYSU has maintained a research program to develop pawpaw into a new high-value tree fruit crop. With its high tolerance for many native pests and diseases, pawpaw shows great potential as a crop for organic and sustainable production. The objectives of KYSU's pawpaw research program include: 1) variety trials; 2) development of new or improved methods of propagation; 3) collection, evaluation, preservation, and dissemination of germplasm; and 4) sharing of information on pawpaw with scientists, commercial growers and marketers, and the general public. To aid in dissemination of information on pawpaw, a web site has been developed (http://www.pawpaw.kysu.edu) that includes information on current and past pawpaw research at KYSU and information on the PawPaw Foundation. On this site, there are a selected bibliography of publications on pawpaw and related species; pawpaw recipes and nutritional information; a guide to buying and growing pawpaws; photos of pawpaw trees, flowers and fruit; and links to other web sites with pawpaw information. In the future, the site will include results from the pawpaw regional variety trials and the database for the National Clonal Germplasm Repository for Asimina spp., located at KYSU. The pawpaw information web site will be an increasingly useful aid in the introduction of pawpaw as a new, potentially high-value, tree fruit crop.

Kirk W. Pomper, Desmond R. Layne and Snake C. Jones

Pawpaw (Asimina triloba) is a native North American tree that has potential as a new fruit crop or for use in landscapes, but until recently, little information has been available to nurseries on containerized production of this species. Pawpaw seedlings develop a strong taproot with a fragile root system, which can be easily damaged upon digging; therefore, most nurseries propagate trees in containers. Pawpaw seed requires stratification for optimal germination and seed is sensitive to desiccation. The seed also cannot tolerate freezing temperatures [<-15 °C (5.0 °F)]. A well-aerated potting substrate with a high sphagnum peat moss component (>75% by volume), cation exchange capacity, and water holding capacity can be used effectively in container production. Tall containers should be used to accommodate the developing taproot of seedlings. The slow-release fertilizer Osmocote 14-14-14 (14N-6.1P-11.6K) incorporated into Pro-Mix BX potting substrate can be used effectively as the sole fertilizer source at a treatment rate of 2.22 kg·m-3 (3.742 lb/yard3) in containerized pawpaw production. It can also be used at a lower rate of 0.81 kg·m-3 (1.365 lb/yard3) when supplemented with weekly applications of 500 mg·L-1 (ppm) of Peters 20-20-20 (20N-8.78P-16.6K) liquid-feed fertilizer. Bottom heating [32 °C (89.6 °F)] of container-grown pawpaw seedlings results in greater lateral and total root dry weight than in seedlings grown at ambient temperature [24 °C (75.2 °F)], which could increase the rate of establishment of seedlings in the field. Bottom heating of container-grown pawpaw seedlings could decrease both the time to produce a saleable plant and the cost of heating greenhouses. Growth of containerized pawpaw seedlings is enhanced by low to moderate shading with polypropylene shade fabric (28% or 51%) outdoors and low shading (33%) in the greenhouse, in a manner typical of that reported for other shade-preferring plants. Low to moderate shading of pawpaw seedlings grown outdoors greatly increases leaf number, total leaf area, and total plant dry weight compared to nonshaded seedlings, suggesting that commercial nurseries can improve production of containerized pawpaw seedlings using a shading regime outdoors.

Kirk W. Pomper, Snake C. Jones and Eddie B. Reed

The pawpaw [Asimina triloba (L.) Dunal] is a native American tree fruit with potential in edible landscapes and as a new fruit crop. A split-plot experiment (main plot: fertilizer level and subplot potting medium) was conducted in the greenhouse to identify the best growing medium for production of pawpaw seedlings. Seeds were sown in rootrainers containing one of the following media: 1) Promix (control); 2) 6 pine bark:1 mason sand (v/v); 3) 1 mason sand: 1 sphagnum peat; and 4) 4 pine bark:1 mason sand:1 sphagnum peat. When seedlings had at least two to three leaves, weekly fertigation of seedlings began, using 0, 250, or 500 ppm Peters 20N-20P-20K. Germination rate at 10 weeks was similar in all media, at about 80%. The plants were destructively harvested 10 weeks after imposition of fertigation treatments. Both potting media and fertigation influenced leaf number and height; however, there was a significant interaction between these main effects. Leaf number and height for plants in medium 3 were similar to those of the control (medium 1), at about 11 leaves and 18-cm plant height, respectively, at 500 ppm fertigation. Plants in media 2 and 4 were about half as tall and had about half as many leaves as control medium plants at 500 ppm fertigation. Plant leaf area and biomass data will be discussed.

Kirk W. Pomper, Snake C. Jones and Eddie B. Reed

In an effort to determine the optimal light level for growing pawpaw [Asimina triloba (L.) Dunal] seedlings outside, seedlings were germinated in a greenhouse until the two- to three-leaf stage, at which time they were placed outside and shade treatments were imposed that reduced incident light intensity by 30%, 55%, 80%, and 95%. Control seedlings were left unshaded outside. A randomized block design was used, with 20 replicate seedlings in each experimental treatment per block. Plants were destructively harvested 11 weeks after the start of the experiment. After 11 weeks, the height and number of leaves per seedling were about 35% higher with light to moderate shading (30%, 55%, and 80%) than in control (unshaded) seedlings. Shoot and leaf dry weights of seedlings grown in 30%, 55%, and 80% shade were almost 2-fold greater than control plants. Root dry weight of seedlings in 30% and 55% shade was 2-fold higher than in control plants. Total plant biomass was greatest in the 30%, 55%, and 80% shade treatments, about 2-fold higher than control plants. Total leaf area per seedling increased significantly with up to 80% shading. Seedlings growing under 95% shading had fewer and smaller leaves and reduced biomass production compared to control plants. There was a trend for shaded plants to display a higher leaf chlorophyll content than control plants. Overall, the best seedling growth was achieved in the 30% and 55% shade treatments outdoors.

Kirk W. Pomper, Snake C. Jones and LaTeasa Barnes

Pawpaw is a native American tree fruit that has great potential as a new commercial crop. The USDA National Clonal Germplasm Repository for Asimina sp. is located at Kentucky State Univ. (KSU); therefore, germplasm collection and storage are important components of the research program. Recalcitrant seeds do not tolerate desiccation, have a relatively short period of viability, and tend not to tolerate subfreezing temperatures. Since pawpaw seed shows a moderate level of recalcitrance, the objectives of this experiment were to determine which storage temperatures (20, 5, -15, and -70 °C for 8.5 weeks) would maintain viable seed, and whether prior seed stratification (5 °C for 17.5 weeks) would influence survival at the various storage temperatures. Seeds were placed in ziplock bags in moist peat moss and subjected to the range of storage temperatures either before or after stratification. After storage and stratification treatments, seed germination rate was examined for 10 weeks at 25 °C on moist filter paper in petri dishes. Both stratification and storage temperature significantly affected seed germination rate. Seeds did not germinate after storage at subfreezing temperatures, regardless of stratification treatment. The best germination rate, ≈70%, was obtained with stratification followed by storage at 20 °C. However, for long-term storage of viable nongerminating pawpaw seed, stratification followed by storage at 5 °C would be most appropriate. Subfreezing storage temperatures were found to be lethal to pawpaw seeds.

Kirk W. Pomper, Desmond R. Layne and Snake C. Jones

The North American pawpaw [Asimina triloba (L.) Dunal] has great potential as a fruit crop or as a landscape plant. The influence of incident irradiance on pawpaw seedling growth and development in containers was examined in the greenhouse and outdoors. Root spiraling can be a problem for container-grown pawpaw seedlings; therefore, the influence of paint containing cupric hydroxide [Cu(OH)2] at 100 g·L-1 applied to the interior of containers on plant growth was also examined in a greenhouse environment. In pawpaw seedlings grown outdoors for 11 weeks, low to moderate shading levels of 28%, 51%, or 81% increased leaf number, total leaf area, and total plant dry weight (DW) compared to nonshaded seedlings. A shading level of 81% decreased the root to shoot ratio by half compared to nonshaded plants. Shading of 98% reduced leaf number, leaf size, and shoot, root, and total plant DW. Shading increased leaf chlorophyll a and b concentrations for pawpaw seedlings grown outdoors, while it decreased average specific leaf DW (mg·cm-2). In a separate greenhouse experiment, pawpaw seedlings subjected to shade treatments of 0%, 33%, 56%, 81%, or 98% did not respond as greatly to shading as plants grown outdoors. Greenhouse-grown plants had greater total and average leaf area under 33% or 56% shading than nonshaded plants; however, shading >56% reduced root, shoot, and total plant DW. Total shoot DW was greater in greenhouse grown plants with 33% shading compared to nonshaded plants. Pawpaw seedlings in control and most shade treatments (33% to 81%) in the greenhouse environment had more leaves and greater leaf area, as well as larger shoot, root, and total plant DW than seedlings in similar treatments grown outdoors. The greenhouse environment had a 10% lower irradiance, a 60% lower ultraviolet irradiance, and a significantly higher (1.23 vs. 1.20) red to far-red light ratio than the outdoors environment. Treatment of container interiors with Cu(OH)2 decreased total and lateral root DW in nonshaded seedlings, and it adversely affected plant quality by causing a yellowing of leaves and reduction of chlorophyll levels by the end of the experiment in shaded plants. Growth characteristics of pawpaw seedlings were positively influenced by low to moderate shading (28% or 51%) outdoors and low shading (33%) in the greenhouse. Seedlings did not benefit from application of Cu(OH)2 to containers at the concentration used in this study. Commercial nurseries can further improve production of pawpaw seedlings using low to moderate shading outdoors.

Marketed as a fresh fruit, the pawpaw (Asimina triloba) has a short shelf life, only 2-3 days at room temperature and up to 3 weeks with refrigeration. There is commercial processing potential for pawpaw pulp in juices, ice cream, yogurt, baked goods, and other products. Consumer acceptability of such products needs to be investigated. One hundred and five attendees of the 2nd Annual Pawpaw Field Day at Kentucky State University (KSU), Frankfort, Ky., participated in a tasting of pawpaw products; 56% of tasters were male; 76% were over 40 years of age; 72% of tasters had eaten pawpaw previously. Each item was rated on a scale from 1 = liked it extremely to 7 = disliked it extremely. Pawpaw ice cream was the best-received item (55% of tasters liked it extremely), followed by pawpaw cake with lemon icing, liked extremely by 45%. The pawpaw/grape juice drink was liked extremely by 31% of participants. Three alternative recipes for pawpaw butter were presented; the plain pawpaw butter was liked extremely by 26% of tasters; pawpaw butter prepared with lemon and grape juice was liked extremely by 11%, while the version prepared with orange and lemon was liked extremely by only 8%. Two versions of pawpaw custard were presented. The custard prepared from ripe, mild-fl avored fruit was liked extremely by 42% of tasters, while the custard prepared from mixed under-ripe, over-ripe and bruised fruit was liked extremely by only 16%. Ratings by persons unfamiliar with pawpaw fl avor were significantly lower (P < 0.05) only for the two pawpaw custards; tasters age 40 years or younger gave significantly higher ratings for pawpaw ice cream (P < 0.05) and significantly lower ratings for both pawpaw custards (select, P < 0.05 and mix, P < 0.01) and the pawpaw/grape juice drink (P < 0.05).

The pawpaw [Asimina triloba (L.) Dunal] is a native plant found mainly in the southeastern and eastern United States, and its fruit has great potential as a new high-value crop in these regions. Although there are ≈45 named pawpaw cultivars, breeding for improvement of specific traits, such as fruit size and quality, is desirable. Our long-term goal is to utilize molecular marker systems to identify markers that can be used for germplasm diversity analyses and for the construction of a molecular genetic map, where markers are correlated with desirable pawpaw traits. The objective of this study was to identify random amplified polymorphic DNA (RAPD) markers that segregate in a simple Mendelian fashion in a controlled A. triloba cross. DNA was extracted from young leaves collected from field-planted parents and 20 progeny of the cross 1-7 × 2-54. The DNA extraction method used gave acceptable yields of ≈7 μg·g-1 of leaf tissue. Additionally, sample 260/280 ratios were ≈1.4, which indicated that the DNA was of high enough purity to be subjected to the RAPD methodology. Screening of 10-base oligonucleotide RAPD primers with template DNA from the parents and progeny of the cross has begun. We have identified two markers using Operon primer B-07 at 1.1 and 0.9 kb that segregate in a simple Mendelian fashion in progeny of the 1-7 × 2-54 cross. Other primers and controlled crosses will also be screened.

The pawpaw [Asimina triloba (L.) Dunal.] is a tree fruit native to many areas of the southeastern and mid-western United States. Kentucky State University (KSU) is designated as a satellite repository for Asimina for the U.S. Department of Agriculture (USDA), National Plant Germplasm System (NPGS). An assessment of the level of genetic diversity in cultivated pawpaw would assist in development of the future germplasm repository collection strategies for cultivar improvement. The objectives of this study were to identify intersimple sequence repeat (ISSR) markers that segregate in a simple Mendelian fashion and to use these markers to assess genetic diversity in 19 pawpaw cultivars. Leaf samples from the 34 progeny of controlled crosses (1-7-1 × 2-54 and reciprocal) and the parents were collected, DNA was extracted, and subjected to the ISSR methodology using the University of British Columbia microsatellite primer set #9. Seven primers yielded 11 Mendelian markers with either a 3:1 or 1:1 ratio that was confirmed by chi-square analysis. Analysis of genetic diversity using 10 of the ISSR markers from 19 pawpaw cultivars revealed a moderate to high level of genetic diversity, with a percent polymorphic loci P = 80 and an expected heterozygosity He = 0.358. These diversity values are higher than those reported for cultivated pawpaw using isozyme or randomly amplified polymorphic DNA (RAPD) markers, indicating that the ISSR marker methodolgy has a higher level of discrimination in evaluating genetic diversity in pawpaw and/or pawpaw has greater levels of genetic diversity than previously found.